A principal concern of current research is the mechanism by which epidermal hyperplasia is produced in skin regeneration, tumorigenesis, and in many skin diseases, such as psoriasis. We have developed a model system for producing epidermal hyperplasia by abrading mouse skin. We find that regenerative epidermal hyperplasia is characterized by a large accumulation of rRNA relative to the accumulation of protein and DNA, and that upon regression of the epidermal hyperplasia rRNA levels return to normal. The purpose of this investigation is to determine the mechanism by which the marked accumulation of rRNA occurs in epidermal hyperplasia, by studying the contribution of rRNA synthesis to the accumulation of rRNA as well as to the return to normal of rRNA levels. This will be done by studying the incorporation of (3H)orotic acid into rRNA of the cytoplasmic ribosomes from the hyperplastic epidermis, and determining autoradiographically the distribution of (3H)orotic acid into the epidermal cells of each of the layers of the hyperplastic epidermis. We also will study the incorporation of (3H)orotic acid into the RNA and rRNA precursor pools. The kinetics of rRNA synthesis will be related to the kinetics of DNA synthesis during the production and regression of the epidermal hyperplasia, by studying the kinetics of incorporation of (3H) thymidine into epidermal DNA, both by scintillation spectroscopy, and autoradiography.